A known system for ejecting ink to form images on a moving web of media material is shown in FIG. 3. The system 10 includes a web unwinding unit 14, a web cleaner 18, a pre-heater roller 22, a plurality of marking stations 26, a turn roller 30, a temperature leveling roller 34, and a spreader 38. In brief, the web unwinding unit 14 includes an actuator, such as an electrical motor, that rotates a web of media material in a direction that removes media material from the web. The media material is fed through the web cleaner 18 along a path formed by the pre-heater roller 22, turn roller 30, and leveling roller 34 and then through the spreader 38 to a rewinder 40. The web cleaner 18 removes debris and loose particulate matter from the web surface to be printed and the pre-heater roller 22 is heated to a temperature that transfers sufficient heat to the media material for optimal ink reception on the web surface as it passes the marking stations 26. Each of the marking stations 26A, 26B, 26C, and 26D in FIG. 3 includes two staggered full width printhead arrays, each of which has three or more printheads that eject ink onto the web surface. The different marking stations eject different colored inks onto the web to form a composite colored image. In one system, the marking stations eject cyan, magenta, yellow, and black inks for forming composite colored images. The surface of the web receiving ink does not encounter a roller until it contacts the temperature leveling roller 34. The temperature leveling roller 34 modifies the temperature of the web for both any inked and non-inked portions and reduces any temperature differences between them. After the temperature leveling, the ink is heated by heater 44 before the printed web enters the spreader 38. The spreader 38 applies pressure to the ejected ink on the surface of the web to smooth the roughly semicircular ink drops on the surface of the web and to encourage ink fill with the different colors and present a more uniform image to a viewer. The web material is then rewound by the rewinder 40 for movement to another system for further processing of the printed web.
This system 10 also includes two load cells, one of which is mounted at a position near pre-heater roller 22 and the other is mounted at a position near the turn roller 30. These load cells are devices that measure the tension on the web monitored by the load cell. Each of the rollers 22, 30, and 34 has an encoder mounted on the roller. These encoders may be mechanical or electronic devices that measure the angular velocity of a roller monitored by the encoder. In a known manner, the angular velocity measured by an encoder may be converted to a linear measurement of the web velocity moving over the roller. The angular velocity signals generated by the encoders and the tension measurement signals generated by the two load cells are coupled to a controller 60. The controller 60 is configured with I/O circuitry, memory, programmed instructions, and other electronic components to implement a web printing system that generates the firing signals for the printheads in the marking stations 26. The term “controller” or “processor” as used in this document refers to a combination of electronic circuitry and software that generates electrical signals that control a portion or all of a process or system. The controller 60 may implement either a single reflex or a double reflex registration system to time the delivery of firing signals to printheads in a print zone of a web printing system. “Double reflex registration system” refers to a system that uses the angular velocity signals corresponding to the rotation of two rollers to compute the web velocity at a printhead positioned between the two rollers. A single reflex registration system refers to a system that uses the angular velocity signals corresponding to the rotation of only one roller to compute a linear web velocity that is used to predict web positions and timing in a print zone.
The system 10 may also include an image-on-web array (IOWA) sensor 68 that generates an image signal of a portion of the web as it passes the IOWA sensor. The IOWA sensor 68 may be implemented with a plurality of optical detectors that are arranged in a single or multiple row array that extends across at least a portion of the web to be printed. The detectors generate signals having an intensity corresponding to a light reflected off the web. The light is generated by a light source that is incorporated in the IOWA sensor and directed toward the web surface to illuminate the surface as it passes the optical detectors of the IOWA sensor. The intensity of the reflected light is dependent upon the amount of light absorbed by the ink on the surface, the light scattered by the web structure, and the light reflected by the ink and web surface. The image signal generated by the IOWA sensor is processed by an integrated registration color controller (IRCC) to detect the presence and position of ink drops ejected onto the surface of the web at the IOWA sensor. Any suitable optical sensor or sensors that can be configured to generate image data for a portion of a moving web and any ink on the web may be used to generate image data for registration analysis.
As noted above, the controller 60 uses the angular velocity measurements from the encoders at the rollers and may also use tension measurements from the two load cells to compute web velocities at the rollers 22, 30, and 34. These velocities enable the controller to determine when a web portion printed by one marking station, station 26A, for example, is opposite another marking station, station 26B, for example, so the second marking station can be operated by the controller 60 with firing signals to eject ink of a different color onto the web in proper registration with the ink already placed on the web by a previous marking station. When the subsequent marking station is operated too soon or too late, the ejected ink lands on the web at positions that may produce visual noise in the image. This effect is known as misregistration. Accurate measurements, therefore, are important in registration of different colored images on the web to produce images with little or no visual noise.
Accurate angular velocity measurements are important for determining the linear velocity of the web at a particular position and the timing of the firing signals correlated to the linear web velocity. In previously known image registration systems, a constant diameter is used for each roller that is monitored by an encoder to generate an angular velocity signal, which is used to compute a linear web velocity. Assuming that the diameter of a roller remains constant may lead to inaccuracies in web velocity calculations. The inaccuracy may be particularly troublesome in heated rollers. These rollers include a heating element that is mounted within the roller or proximate the roller to heat the roller to a temperature above the ambient temperature of the environment of the roller. The heated roller may be used for such purposes as preconditioning the web for printing or the like. When the roller is heated, the material forming the rotating cylinder of the roller expands. This expansion is particularly apparent in rollers having cylinders formed from metal, such as aluminum or stainless steel. The changes in the diameter of the roller cylinder may be significant enough to affect the accuracy of the velocity computed for the web and the timing of the firing signals for the printheads that eject ink as the web passes by the printheads.